Optical modulators may be used to electrically modulate light from a light source for optical communication and optical data transmission. The efficiency of an optical modulator (i.e. the aspects of the drive voltage or power requirement of the optical modulator) is fundamentally determined by the electro-optic (EO) materials used to construct the modulator.
Silicon (Si) based optical modulators typically include a PN junction. These modulators rely on free carrier dispersion effects (induced via injection, depletion, accumulation or inversion of carriers) to modulate both the real and imaginary parts of the refractive index of the p-type and n-type silicon near the PN junction. In free carrier absorption modulators, changes in the optical absorption within the modulator are directly transformed into a light output intensity modulation.
Si based optical modulators in forward bias operation typically have limited operating speeds (in the range of 10-50 Mb/s). This operation speed is limited due to the resistance-capacitance time constant product, where the capacitance of the modulator becomes very large due to the reduction in the depletion layer width of the PN junction in forward bias.
To achieve high speed operation (i.e., speeds greater than 10 Gb/s) the PN junctions of silicon based modulators are used in reverse bias to prevent the high capacitance of forward bias operation. This results in modulators requiring an excessive length (several millimeters) and high drive voltages (Vπ>5 V). Improvements in the efficiency of prior art Si based modulators have come from scaling the waveguide dimensions of the modulator to sub-micron geometries and from introducing an insulator between the P and N silicon regions. This insulator allows for accumulation of charge without the increased capacitance of a forward biased junction. However, additional improvements in these areas will be limited as the optical confinement of the PN junction waveguides will degrade as the dimensions are scaled down further, resulting in a drop in efficiency.
III-V semiconductor based optical modulators rely on field based modulation that may achieve up to 50 times the efficiency of Si based modulators at a given length. Unfortunately, there is a tradeoff in III-V based optical modulators as the design has traditionally been either a structure with high electro-optic efficiency, high optical propagation loss and a short length, or a structure with low electro-optic efficiency, low optical propagation loss and a long length.
Thus, both prior art III-V modulator designs have similar bandwidth and drive voltages, and high electro-optic efficiency results in shorter III-V modulators. This limitation stems from the fact that in order to confine the applied electric field maximally to the optical field, prior art III-V based modulators require a PIN (p-type semiconductor-intrinsic semiconductor-n-type semiconductor) junction. P-type III-V semiconductor materials produce an associated optical and microwave loss (due to the p-dopants). If a III-V based modulator uses an NIN (n-type semiconductor-intrinsic semiconductor-n-type semiconductor) junction or Schottky junction to avoid this optical loss, the electro-optic efficiency is degraded due to the additional voltage being dropped across buffer materials that are required to prevent optical loss from the metallic layers. These buffer layers further act as insulating regions that block current flow.
With regards to the cost and size of an optical modulator, silicon materials are easily processed with current techniques, high quality silicon materials are readily available for reasonable a cost, and complex VLSI silicon electronic circuits are readily available. However, silicon-based modulators (or other photonic devices such as lasers) are not as efficient at light emission or absorption as their III-V based counterparts.
Attempts in the prior art have been made to utilize both materials to create photonic devices integrated with Complementary Metal Oxide Semiconductor (CMOS) integrated circuits; however, these attempts have been limited in that electro-optic modulation stems solely from the III-V material, while utilizing the silicon material solely for passive optical wave guiding and/or driving circuitry. Therefore, prior art silicon/III-V photonic integration is limited in that it relies solely on the electro-optic modulation within the III-V region, and thus reduces the potential efficiency and requires the use of p-type dopants within the III-V materials which significantly increase optical and microwave propagation losses.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.